Oxyanions of selenium have been identified as environmental toxins in drainage waters from irrigated agricultural soils that contain selenium. Presser, T. P., et. al. 1984, U.S. Geological Survey Water Resources Investigations Report 85-4220. This environmental problem is common in many irrigated regions of the western United States of America. Sylvester, M. A., et. al.1988, On Planning Now for Irrigation Drainage Studies, Proc. Amer. Soc. Civil Engin., New York, p. 665-667. Methods for removal of selenium oxyanions offer the prospect of restoring water or soil quality while allowing for the continuation of irrigated farming and have therefore received attention.
Baldwin et. al., U.S. Pat. No. 4,405,464, discloses a purely chemical process for reducing selenate to a lower oxidation state by treating aqueous selenate solutions with iron under pH conditions favoring the formation of a precipitate of an iron hydroxide and some amount of elemental selenium. Baldwin et. al. U.S. Pat. No. 4,405,464 does not, however, suggest the use of any biological process in the production of elemental selenium from waste waters containing selenium oxyanions.
Baldwin et. al., U.S. Pat. No. 4,519,913, discloses a process for the removal of selenium from contaminated water using microorganisms of the genus Clostridium. According to the disclosure, the process is run under anaerobic conditions and selenium is produced in a recoverable form. The process, however, is suited to industrial or mining waste water and does not provide for any way of maintaining the process using waste waters that contain chemical species that inhibit selenate reduction to selenium. In particular Baldwin et. al., U.S. Pat. No. 4,519,913, fails to disclose any method for coping with nitrate ion which inhibits the reduction of selenate to selenium.
Downing et. al., U.S. Pat. No. 4,725,357 also discloses a microbial process for the removal of selenium and suggests the desirability of removing nitrate to a level of 5 milligrams per liter or less prior to the treatment of the selenium- containing waste water. Downing et. al. however, does not specify an operational range below 5 milligrams per liter nitrate where practical levels of removal of selenate can occur. Furthermore, Downing et. al. does not suggest that the nitrate is removed by biomass assimilation of the nitrate under aerobic conditions. In addition, although Downing et. al. discloses that exogenous nutrients may have to be supplied to the microorganisms in the reactor where selenate is reduced, there is no suggestion that the biomass used to assimilate nitrate can also be processed and circulated into the reactor to serve as the nutrient source for the microorganisms in the reactor. The requirement of using exogenous nutrients (including electron donors) as discussed in Downing et. al. adds substantial cost to running the process. In addition, although Downing et. al. discloses that microorganisms are useful for the removal of selenate, there is no suggestion that such microorganisms in fact respire selenate. Downing et. al. also teaches that it is not necessary to seed the reactor with selenate utilizing organisms and that optimal selenate-utilizing microorganisms can be obtained in the reactor merely by populating the reactor with selenate utilizing organisms from any source.
Kauffman et. al., U.S. Pat. No. 4,519,912 also discloses a microbial process for the removal of selenium ion from waste waters, however the process according to the disclosure requires the action of Desulfovibrio and other sulfate reducing microorganisms in the presence of sulfate to produce hydrogen sulfide. The process of Kauffman et. al., however cannot be run at high concentrations of selenate that may exist in some agricultural waste waters after evaporation, because such high concentrations of selenium are toxic to the sulfate utilizing microorganisms. The process of Kauffman et. al. is not independent of sulfate metabolism.
Zehr, J. P., and Oremland, R. S. 1987, Reduction of Selenate to Selenide by Sulfate-Respiring Bacteria: Experiments with Cell Suspensions and Estuarine Sediments. Appl. Environ. Microbiol. 53:1365-1369., disclosed that washed cell suspensions of Desulfovibrio desulfuricans subsp. aestuarii were capable of reducing nanomolar levels of selenate to selenide as well as sulfate to sulfide. More importantly, these reductive process were inhibited by 1 mM selenate. Furthermore, the addition of 1 mM sulfate decreased the reduction of selenate and enhanced the reduction of sulfate, demonstrating that increasing concentrations of sulfate inhibited selenate reduction but enhanced sulfate reduction rates. Measurements of the reduction of picomolar amounts of selenate by sulfate reducing microorganisms indicated that less than 0.1% of the selenate is converted to selenide (Se.sup.2) even in the presence of lactate and H.sub.2. The authors indicate that these observations suggest that a reaction other than direct reduction by sulfate reducing bacteria occurs. These results cast doubt upon the teaching of Kaufmann et. al. mentioned above.
Oremland et. al., 1989., Selenate Reduction to Elemental Selenium by Anaerobic Bacteria in Sediments and Culture: Biogeochemical Significance of a Novel, Sulfate-Independent Respiration. Appl. Environ. Microbiol. 53:1365-1369., disclosed a microbial process that removes oxyanions of selenium from water through microbial respiration and produces elemental selenium under anaerobic conditions. Oremland et. al. also discloses that the removal of selenate under these conditions is substantially inhibited by 10 mM nitrate.